In cellular networks, radio access base stations must use a very accurate time base as a reference for their frequency generation circuitry and other components. In order to achieve the required degree of accuracy, which is typically on the order of 0.05 parts per million (ppm), this reference must be even more accurate which thus requires specialized hardware. Various synchronization schemes include synchronizing with an atomic clock, using a frequency derived from a dedicated backhaul connection (e.g., deriving a frequency reference from a T1, E1, or fiber optic cable in conjunction with a Stratum-1 clock as a reference), or using a frequency reference provided by a Global Positioning System (GPS) receiver. These reference schemes are practical in larger base stations where cost sensitivity is low and a fixed line backhaul is standard.
A new type of base station providing personalized coverage has become attractive to some carriers for subscribers' homes and small offices rather than covering large districts of urban or sub-urban areas. These new base stations are known as femtocells, and are characterized by much smaller coverage areas, consumer-grade packaging and price-points, and the use of consumer internet protocol (IP) connections using various common wireline technologies. These wireline technologies, may include, but are not limited to: DSL, DOCSIS, powerline, and/or coaxial cable. The lack of a fixed line backhaul and extreme cost sensitivity of these femtocells require different synchronization schemes than larger cells use. Additionally, traditional GPS synchronization may not work with femtocells as they are typically installed indoors where a GPS receiver cannot receive a signal from the GPS satellite system that is required to provide the high accuracy frequency reference.
One popular synchronization scheme for femtocells is the IEEE 1588 standard which addresses this issue quite well under normal circumstances. Timing packets are provided over the IP network and the femtocells effectively use the arrival events of these packets as a timebase after filtering out jitter and other errors created by the IP network. A local oscillator in the femtocell is typically used as a local timebase, and this local timebase is compared with the packet-driven timebase. The frequency of the local oscillator is adjusted so that the two timebases are synchronized.
When the IP connection is stable and the network is lightly loaded, this packet-based synchronization scheme works quite well. But as network loading increases and creates more packet jitter and other errors such as packet loss, the derived synchronization error can increase and may cause the femtocell carrier frequency to exceed a specified error tolerance. In one extreme case, the consumer IP network may be out of service completely and the timing packets would not be available to keep the femtocell adjusted properly.
If a femtocell is using a packetized time base reference signal sent over a network, the femtocell must rely on a local oscillator if the network is unavailable. In this situation, prior art systems have relied on ovenized oscillators to keep an accurate frequency reference. However, these ovenized oscillators are relatively expensive and require calibration. Less precise oscillators encounter frequency drift as a result of manufacturing variations or environmental factors such as temperature, humidity, or the age of the oscillator.
In general, the local oscillator is used to generate a carrier frequency, which in turn is used by a radio access node to generate a transmission signal for wireless communications. Within the transmission signal, each wireless resource, e.g. a resource block or channel, is allocated a unique frequency. When the carrier frequencies for neighboring radio access nodes are synchronized, the resource blocks are transmitted within their specified values, and there is little interference between wireless resources.
However, when a femtocell carrier frequency exceeds the specified error tolerance, a downlink transmission to a mobile device will be outside an allocated frequency range. The mobile device may tolerate this error and receive the downlink. When the mobile device sends an uplink transmission back to its associated radio access node, energy from adjacent frequency resource blocks that are used by neighboring cells will spill into the frequency resource blocks being used by the mobile device and femtocell. When this situation occurs it is an indication of carrier frequency error, and there will be significant interference in the received radio frequency band.
Presently, there is a need for improved systems and methods that facilitate the femtocell taking corrective action in the event of a carrier frequency error. It would be beneficial if the femtocell could detect its own frequency error based on the indications of other parts of the network. To date, systems and methods for addressing carrier frequency error have focused on ensuring there is no deviation in the carrier signal. This approach has worked in large macrocell, microcell, and picocell radio access nodes, but does not scale well for use in a femtocell eNodeB base station.